The transmission of information over very high-speed links requires, to enhance the transmission characteristics of the signal, that data strings to be forwarded have to be first encoded by the transmitting side. A particular desirable characteristic to acquire is that, irrespective of the information to be transported, the binary transmitted signal remains DC balanced. Also, transmitted signal must have enough transitions so as the receiver side can reliably recover a clock and can operate without any drift that would possibly result from long sequences of transmitted 1's or 0's.
A standard technique to achieve this objective is to scramble the transmitted data. Thus, various scramblers are specified by telecommunications standards. The one of the IEEE 802.3ae physical coding sublayer (PCS) broadly referred to as 10 Gb Ethernet (10 GbE), using a transmission coding technique known as 64B/66B, is based on polynomial: x58+x39+1.
On the other hand, the bit error rate (BER) of high-speed transmission links cannot generally be specified at a better rate than 10−2. Being given that any modern data communications equipment is now required to reach aggregate data throughput in a tera (1012) bits per second (Tbps) range then, hundredths of links (link speed are typically in a 2.5 to 12.5 Giga or 109 bits per second range) must be used irrespective of the actual implementation of a machine. Hence, it can easily be seen that errors are thus susceptible to occur at a rate well in excess of one every second. Even though link BER might be, in practice, better than what can be specified, designers of such data communications equipment such as Tbps routers and switches, are now forced to consider the use of forward error correction (FEC) a standard technique in which redundant bits of information are transmitted so a receiver can correct errors without requiring re-transmission.
Unfortunately, scrambling and FEC are somehow incompatible. It is well-known that scramblers multiply the number of error bits after de-scrambling. For example, 10 GbE scrambler, which has three terms, creates three errors after decoding for each real error occurring on a transmission link. Moreover, these three errors span on 59 bits corresponding to the degree of the polynomial. While a single error can easily be corrected with a simple FEC code e.g., a Hamming code, requiring few extra bits, much more redundancy is needed to handle the bit error spreading as a result of the scrambling. Also codes to use in this case, such as BCH codes, are complex to decode.